Long-term smoking is the leading preventable cause of cancers and of premature mortality, with approximately one billion smoking-related deaths projected globally, based on present trends, in the current century. This is the re-revised (A2) competing continuation of a genetic epidemiologic collaboration to identify genes that influence risks for heavy smoking and nicotine dependence. During the first five-year funding period, QTL linkage analyses using 10cM microsatellite marker genome scan data have identified a strong linkage signal in both Australian and Finnish sibships, for our primary heaviness-of-smoking (HoS) phenotype, on Chr 22q12 (in updated analyses, multi-point LODs of 3.05 and 3.21 respectively). Genotyping of two additional microsatellite markers flanking the linkage peaks, in both Finnish and Australian genome-scan samples, yielded a combined multipoint LOD of 5.21 (genome-wide significance p=.006) at 25cM. This competing continuation seeks to identify the gene or genes at 22q12 which are contributing to HoS using the efficiency of existing data and DNAs and large informative samples, with complementary strengths, that were drawn from the same populations in which the original linkage signal was obtained. This goal will be achieved by (i) conducting a genetic association study, with high-throughput genotyping of 1436 SNPs across a 1-LOD support interval within our primary linkage region in 1000 unrelated Finnish male heavy smokers (ATBC chemoprevention trial participants who smoked 40+ cigarettes per day) and 1000 Finnish male population controls (Health2000 participants) (Aim 1: 'Finnish series I'; 'stage 1 genotyping'); (ii) conducting follow-up genotyping of SNPs found nominally significant at stage 1 in two separate series: (a) an additional 2000 unrelated heavy smokers (who smoked 30+ cigarettes per day) and 2000 population controls ('Finnish series II'); and (b) a family-based series of 4429 siblings who are smokers (55 percent female) from a community-based series of Australian large sibships ('BIGSIB' sample) ascertained solely on the basis of large sibship size from the Australian twin panel, from the same cohorts as the Australian linkage families (Aim 2: 'stage 2 genotyping'). Combining Finnish series I and II will ensure adequate power to detect quite modest QTL effect sizes; confirmation of nominal stage 1 associations in the Australian series will ensure that we are able to detect genetic effects that may be only modest in male long-term smokers, but more potent in female smokers. For genes with SNPs confirmed to be associated with HoS, we will conduct DNA sequencing in individuals with high-risk and low-risk haplotypes, selected from the original Finnish and Australian linkage families, to attempt to identify variants that might be functional (Aim 3), and will characterize using family-based association approaches the effects of genetic variants (or haplotypes) thus identified on cigarette-smoking and related behaviors (including correlated substance use and other psychiatric disorders), using the original Finnish NAG families and the Australian BIGSIB sample (Aim 4). The importance of genetic influences on risk of becoming a long-term heavy smoker has been known for many years. Understanding the genetic mechanisms by which some individuals are at increased risk should ultimately lead to improved therapies to assist smoking cessation.